A device that uses diffuse reflectance characteristics of near-infrared light to measure the moisture content, protein and the like that are contained in flowing grain is known from Patent Literature 1. In this device, a light source that is arranged along a discharge duct of a farm machine such as a harvester, and is configured to irradiate the flow of cereal grain flowing in the discharge duct, and a detector that is configured to detect light that is scattered and reflected from the cereal grain are disposed in the same housing, and a shielding object is provided at a position at which it separates the detector from the light source that is lined up therewith. In this device, the detector receives light that is projected from the light source to the grain and is returned, but, due to the structural feature thereof, light from the light source is likely to directly enter the detector at the time of measuring the grain.
An optical internal-quality measuring means that evaluates the internal quality of grain that has been threshed and is temporarily stored is known from Patent Literature 2. This optical internal-quality measuring means is provided on a grain tank of a combine harvester, and is configured to irradiate the grain with near-infrared light, analyze an absorption spectrum based on spectroscopic analysis of the transmitted light, and determine the amounts of components such as moisture content, protein, and amylose that are contained in the grains based on a result of the analysis. The optical internal-quality measuring means is provided with: a light source; a measurement probe that guides a measurement light beam from the light source and diffusely-reflected light from the grain; a light-projecting/receiving adapter that irradiates the grain with the measurement light beam guided by the measurement probe, and receives the diffusely-reflected light from the grain to guide the received light to the measurement probe; a spectroscopic measurement unit that measures a spectroscopic spectrum of the diffusely-reflected light guided by the measurement probe; and an arithmetic unit that performs arithmetic processing on the components that are contained in the grains based on the spectroscopic spectrum obtained by the spectroscopic measurement unit. The light-projecting/receiving adapter and the measurement probe are housed in a cover body, and the light source, the spectroscopic measurement unit, and the arithmetic unit are housed in a separate device from the cover body. The measurement probe is constituted by a light-emitting optical fiber and a light-receiving optical fiber. The portions of the light-emitting optical fiber and the light-receiving optical fiber that respectively excludes an entrance end side, on which the measurement light beam is incident, of the light-emitting optical fiber, and an exit end side, from which the diffusely-reflected light exits, of the light-receiving optical fiber are formed coaxially such that the light-receiving optical fiber is located inside the ring-shaped light-emitting optical fiber. The light-projecting/receiving adapter is attached to the front end of the measurement probe, and is constituted by: an outer tubular body; an inner tubular body that is located inside the outer tubular body, and is coaxial with the outer tubular body while being distanced therefrom; and a connecting member that connects the outer tubular body and the inner tubular body.
This optical internal-quality measuring means has a configuration in which the light-emitting optical fiber is used to guide light from the light source to the light-projecting/receiving adapter, and the light-receiving optical fiber is used to guide light from the grain via the light-projecting/receiving adapter to the spectroscopic measurement unit. Furthermore, the light-projecting/receiving adapter is configured to include the outer tubular body and the inner tubular body that is coaxial with the outer tubular body while being distanced therefrom. This causes the problem of high manufacturing costs.